The invention relates to a device for the manufacture of hydrogen bromide by direct combustion of bromine in hydrogen.
U.S. Pat. No. 2,070,263 describes a method for obtaining aqueous solutions of hydrobromic acid which consists, in a first step, in passing hydrogen through liquid bromine maintained at a temperature of between 37xc2x0 C. anxc2x0 42xc2x0 C. in order to form a mixture of bromine and hydrogen gas which is burnt at a temperature of between 600xc2x0 C. and 850xc2x0 C. By working in this way, it is difficult to have an intimate mixture of bromine and hydrogen in stoichiometric amounts on account of the difficulties in rigorously maintaining the temperatures and the thermodynamic equilibria.
An instability of the combustion flame has also been observed, in devices based on the direct combustion of bromine in hydrogen according to an H2/Br2 molar ratio of greater than 1, and for which no technique for mixing the reactants is mentioned, this instability being manifested in particular by strong vacillation of the flame at the burner outlet, going as far as a detachment of the flame (xe2x80x9cblow offxe2x80x9d) from the said burner, which may entail a risk of explosion, a fluctuating quality of the hydrogen bromide gas produced and a risk of the flame returning.
Furthermore, such flames become extended forming cones at the base of which are regions from which the reactants are liable to escape without being burnt.
This disrupts the combustion of the bromine in the hydrogen and results especially in residual bromine in the combustion gases, this being of a nature to bring about a considerable decrease in the lifetime of the burners, limit the range of materials which may be used and degrade the quality of the hydrogen bromide gas, thereby preventing it from being used as a reactant for downstream syntheses (secondary reactions, colorations of the products) or for the preparation of pure hydrobromic acid solutions.
Patent FR 2,365,516 proposes a process which improves the stability of the flame resulting from the combustion of bromine in hydrogen by establishing a helical stream of bromine in a cylindrical chamber, then injecting the hydrogen radially towards the outside in the helical stream of bromine and continuously supplying a flame close to the chamber with the helical stream of bromine and hydrogen.
This process, using a molar excess of hydrogen of 2.6%, leads to an HBr gas containing 300 ppm of bromine by volume, which still gives rise to colorations of the downstream synthesis products as well as the drawbacks mentioned above.
In addition, the complexity of the burner entails a lack of flexibility. Thus, in particular, when it is desired to increase the capacity of said device, several burners are arranged side by side in the same chamber. In such an arrangement, it cannot be avoided that the flame from different burners mounted in parallel will interfere with each other, and furthermore this arrangement is unacceptable with regard to obtaining good distribution of the reactants. This configuration inevitably leads to a lowering in the degree of conversion of the bromine, complicates the control of the cooling of the HBr formed and increases the risks of explosion.
Patent application DE 2,738,744 describes a device for the combustion of bromine in hydrogen which comprises a combustion chamber, characterized by its tube shape and a transverse section which gradually largens from the orifice of the conduit of the said combustion chamber. This device has the drawback of being relatively inflexible at low speed, of being sensitive to flame return and of giving mediocre-quality HBr gas (2000 ppm by weight of bromine).
It has now been found that a device as described later can be used in a process for manufacturing hydrogen bromide by direct combustion of bromine in hydrogen according to the reaction H2+Br2xe2x88x922HBr, the said process consisting in carrying out the following steps successively:
intimately mixing hydrogen and an oxidant in a chamber,
starting a flame at the outlet of the said chamber,
replacing all or part of the oxidant by a stream of prevaporized bromine, in an H2/Br2 molar ratio which is suitable for maintaining a stable flame at the outlet of the said chamber and in a so-called combustion zone, thereby making it possible to obtain complete combustion of the bromine in the hydrogen,
cooling the combustion gases in a cooling zone, then
recovering a flow of hydrogen bromide gas under a pressure at least equal to 1 bar absolute and preferably under a pressure between 1.3 bar absolute and 10 bar absolute and at a temperature not above 125xc2x0 C. and preferably between 40xc2x0 C. and 125xc2x0 C.
According to the present invention, any mixture consisting of a gas which is inert towards the reactants of the reaction for the formation of HBr and an amount of oxygen which is sufficient to lead to a combustible mixture with hydrogen may be used as oxidant. Nitrogen may be used as inert gas. The oxidant is preferably air.
According to the present invention, the oxidant may be partially or totally replaced. In the case where a certain amount of oxidant is maintained, this amount may vary within a wide range which depends in particular on the organic impurities present in the supply stream of bromine gas, the size characteristics of the apparatus and the use downstream of the HBr produced.
Hydrogen and the oxidant are introduced in gaseous form into the chamber. When the flame is started, the oxidant is replaced by bromine gas introduced at a temperature above its boiling point and preferably between 100xc2x0 C. and 160xc2x0 C.
The hydrogen will advantageously be preheated to a temperature not above 150xc2x0 C. and preferably between 60xc2x0 C. and 100xc2x0 C.
The pressure in the chamber is at least equal to 1 bar absolute, preferably greater than 1.3 bar absolute and even more preferably between 1.8 bar absolute and 10 bar absolute.
It would not constitute a departure from the invention if the pressure was slightly below atmospheric pressure.
The H2/Br2 molar ratio is greater than 1. Preferably, the H2/Br2 molar ratio is greater than 1 and less than 1.5. The Applicant has observed that, under the reaction conditions of the present invention, the bromine is consumed instantaneously and almost completely.
The reaction of bromine and hydrogen is accompanied by a release of heat (12.3 kcalories per mole of HBr gas formed) which raises the temperature of the combustion flame to more than 1600xc2x0 under adiabatic conditions.
According to the present invention, the cooling of the hydrogen bromide formed starts from the moment of its formation in the combustion zone as a function of the excess hydrogen and of the cooling of the combustion chamber, and then continues in a cooling zone designed such that the temperature falls gradually in these zones and is in the region of 600xc2x0 C. at the outlet of the cooling zone and is preferably between 40xc2x0 C. and 125xc2x0 C. This outlet temperature can be adapted as a function of the constraints of the downstream devices. The pressure prevailing in the said cooling zone is preferably between 1.3 bar absolute and 10 bar absolute.